This invention relates generally to the alkylation of hydrocarbons in a shell and bare tube type alkylation reactor. More specifically the invention relates to enhancing the hydrocarbon alkylation process by improving the inside film heat transfer coefficient of bare tube type reactors.
The process of alkylating isoparaffins with olefins in the presence of an acid catalyst to produce branched hydrocarbons, known as alkylates, is a well known commercially practiced process. Indeed there are four different commercial methods for carrying out this process. These are: sulfuric acidxe2x80x94autorefrigeration, sulfuric acidxe2x80x94effluent refrigeration, hydrofluoric acid-time tank, and hydrofluoric acid tubular reactor. In the case of sulfuric acidxe2x80x94effluent refrigeration, with which the present invention is concerned, a hydrocarbon effluent is flashed within a heat exchanger to cool the reaction zone. The reactor employed in this process is similar to a large shell-and-tube heat exchanger which is provided with a mixing impeller at the reactant inlet end of the reactor. The exothermic alkylation reaction occurs on the shell side of the reactor while flashing hydrocarbons within the tube provides the requisite cooling. The reactor operating temperature is determined mainly by olefin feed rate, isoparaffin concentration, feed stream inlet temperatures, mixing power, heat transfer area, heat transfer coefficient, and compressor suction pressure. Notwithstanding the foregoing the fixed amount of heat transfer surface area, i.e., the surface area of the bare tubes used in the reactor, is a significant factor contributing to the reactor operating at a temperature that is higher than what might be considered optimum.
In the alkyation reaction the lower the reactor temperature the lower the acid consumption, the greater the yield of alkylate and the better the quality of alkylate. One way the reaction temperature can be lowered is by increasing refrigeration compressor capacity (e.g., lower suction pressure); however, this is costly and therefore other techniques to enhance heat removal to maintain a more desirable reaction temperature condition have been proposed. For example, the use:o enhanced nucleate boiling surfaces to increase the heat transfer film coefficient on the boiling side heat transfer surface is disclosed in U.S. Pat. No. 4,769,511.
As is pointed out in U.S. Pat. No. 5,625,112 use of enhanced boiling surfaces in the operation of some heat exchangers does not always provide a benefit. Indeed U.S. Pat. No. 5,811,625 teaches that enhanced boiling surfaces in an acid catalyzed alkylation process provided little or no benefit. In other words heat transfer tubes coated with an enhanced surface on their inside perform at essentially the same heat transfer rates as bare tubes.
Now it has been discovered, that bare heat transfer tubes fitted internally with inserts capable of increasing turbulence and mixing within the tube result in enhanced heat transfer in the acid catalyzedxe2x80x94effluent refrigerated alkylation process.
In one embodiment, this invention is a process for a alkylating isoparaffinic hydrocarbons with olefinic hydrocarbons in the presence of an acid catalyst. The process includes steps of reacting the isoparaffinic hydrocarbons and olefinic hydrocarbons in the presence of acid catalyst to form alkylate, separating a hydrocarbon mixture into an acid phase and a hydrocarbon phase, reducing the pressure on the hydrocarbon phase to refrigerate and vaporize volatile hydrocarbons and passing the refrigerated hydrocarbon phase through the interior of a plurality of heat exchange tubes. Vaporization of the hydro-carbons in contact with the interior surface effects indirect heat exchange to cool the reaction mixture which is in contact with the exterior of the heat exchange tubes. This invention improves the alkylation process by passing the hydro-carbon phase to heat exchange tubes fitted with inserts capable of increasing turbulence and mixing within the tube whereby heat transfer coefficient is increased.
Additional objects, embodiments, and details of this invention are set forth in the following detailed description of the invention.